One or more embodiments of the present invention relate to a nonvolatile memory device and, more particularly, to a nonvolatile memory device and a method of operating the device, which are capable of checking the state of a memory cell by calculating a threshold voltage distribution of the memory cell based on a threshold voltage distribution of a reference memory cell.
A nonvolatile memory device has the advantages of a random access memory (RAM), enabling the writing and erasure of data, and a read only memory (ROM), retaining data even without the supply of power, and so has recently been widely used for the storage media of portable electronic devices, such as digital cameras, personal digital assistants (PDAs), and MP3 players.
The nonvolatile memory device includes a memory cell array, a row decoder, a page buffer unit, etc. The memory cell array includes a plurality of word lines elongated in rows, a plurality of bit lines elongated in columns, and a plurality of cell strings corresponding to respective bit lines.
Memory cells have varying threshold voltages according to their program states. Ideally, the memory cells have the same threshold voltage according to the state of data to be stored. However, when a program operation is actually performed on the memory cells, the threshold voltages of the memory cells have probability distributions in some regions because of various external environments, such as the device characteristics and the coupling effect.
If such probability distributions are calculated and checked, it is possible to verify whether a corresponding memory device has been programmed with a correct threshold voltage after a program operation has been performed.
To this end, the locations of the memory cells are checked by performing a read operation while increasing a specific voltage supplied to a selected word line on a page basis, and the above operation is repeatedly performed on a number of pages. Accordingly, threshold voltage distributions of the memory cells can be checked. To check the threshold voltage distributions of memory cells on a memory-block basis, a considerable amount of time is required because the above operation must be repeatedly performed on all of the pages.